There is increasing use being made of DC to AC inverters as a source of power for mains appliances, either in remote locations that do not have a connection to a mains supply grid, as part of an un-interruptible power supply which provides mains power in the event of a sudden interruption in the supply of grid connected power, or to connect sources of renewable energy such as wind or solar to the grid. In remote power applications it has been common for inverters to only be switched on when there is a need to operate a mains powered appliance. However, many mains appliances are now designed to operate in a standby mode when not actively being used. An example of this is a video recorder which goes into a low power standby mode while it is waiting to start recording a program. A second example is a television set that can be brought out of standby mode by the power button on its remote control. Both these examples require a continuous supply of power, even when the appliances are not actively being used. In view of this it is becoming common for inverters to be turned on continuously. When this is done the standby power of the inverter itself can become significant, especially in installations where there is a limited amount of power, such as those using solar panels, or in installations where the inverter is run from a storage battery that is recharged periodically and must supply all the installation's power between each recharge. Most of the standby power is the power required to operate the circuitry of the inverter, so this power is also being consumed even when there is power passing through the inverter. Reducing the standby power will improve the efficiency of an inverter, which is an important consideration for inverters that connect sources of renewable energy to the grid.
Most commercially available inverters utilise voltage controlled power switching devices such as MOSFETs or IGBTs. Many use a combination of these two devices. For both of these devices the name of the control terminal or electrode is the gate. The inverters that provide substantially pure sine wave outputs generally use some form of pulse width modulation (PWM) that requires that the power switching devices be turned on and off rapidly, commonly with a PWM switching frequency that is between 20 kHz and 100 kHz. MOSFETs and IGBTs have a significant gate capacitance that must be charged to turn the device on and discharged to turn it off. The power required to charge and discharge the gate capacitance of each device once per PWM cycle forms a significant part of an inverter's standby power. This “gate drive” power becomes even more significant as the switching frequency is increased. However, it is desirable to increase the PWM switching frequency as this allows the size and cost of filter inductors and capacitors in the inverter to be reduced. New voltage controlled switching devices are being developed such as silicon carbide junction FETs. It is likely that these devices will also have significant gate capacitances.
FIG. 1 shows the circuit of a typical DC to AC inverter suitable for converting a source of DC power to a standard mains electrical AC voltage and frequency with a substantially pure sine wave output. The inverter is of a general type well known in the art where a DC to DC converter utilising a high frequency transformer steps up the voltage of the source of DC power to a higher DC voltage, which in turn is converted to a mains voltage AC output by means of pulse width modulation (PWM) followed by low pass filtering. To simplify the drawing, no drive circuits have been shown for the power switching devices. The inverter circuit shown makes use exclusively of MOSFETs, but IGBTs could also be used for some or all of the high voltage power switches.
The DC to DC converter is of a type known as a voltage sourced dual active bridge. Section 4 contains a full bridge of low voltage power switches while section 8 contains a full bridge of high voltage power switches. These two bridges are switched in unison so as to apply a substantially square voltage waveform to the high frequency transformer 6. Power is able to be transferred either from the low voltage reservoir capacitor 3 to the high voltage reservoir capacitor 9, or in the reverse direction. The ratio of the voltage across the low voltage reservoir capacitor to the voltage across the high voltage reservoir capacitor is substantially equal to the turns ratio of the high frequency transformer. When the DC to DC converter first starts operating it is necessary to apply short pulses to the transformer instead of square waves in order to limit the current flowing into the high voltage reservoir capacitor. Once the voltage across this capacitor has reached a value close to the final operation voltage the switching of the devices in the two bridges can be adjusted so that a substantially square wave is applied to the transformer. Thus it is necessary for the gate drive circuits for all the power switching devices in these two bridges to be able to switch the devices on for a range of time intervals from one that is short in comparison to the switching cycle to one that is close to half of the switching cycle.
The PWM operation is performed by two half bridges. The first is contained in section 10. This is a low frequency half bridge where each device is switched on for half a mains cycle then off for half a mains cycle and both devices are switched on and off out of phase with each other. The gate drive circuit required to perform this low frequency switching operation is not covered herein. The second half bridge is shown in section 11 and is a high frequency PWM circuit which produces first a positive half sine wave with a zero reference equal to the negative high voltage rail, followed by a negative half sine wave with a zero reference equal to the positive high voltage rail. The output of the PWM circuit is filtered and applied to one of the inverter output terminals while the other output terminal is connected to the output of the low frequency half bridge, resulting in the voltage generated across the two output terminals being a complete sine wave with both positive and negative excursions. At the start and end of each half sine wave one of the power switching devices in the PWM circuit should be turned on for a complete PWM cycle while the second device needs to be turned off whenever the first device is on. As the half sine wave moves towards its peak the first device is progressiyely turned off for a greater and greater percentage of the cycle while the second device is turned on for matching progressively longer periods. This requires gate drive circuits that can turn power switching devices on for a range of periods from zero to 100% of the PWM cycle. Many gate drive circuits are not able to provide this range of periods. In practice it is possible to produce acceptable sine waves if the power switching devices can be switched on for time intervals that range from slightly greater than zero to slightly less than 100% of the PWM cycle. For example, if the PWM cycle is 10 microseconds long, then a suitable minimum on time is in the order of 100 nanoseconds and a suitable maximum on time is in the order of 9.9 microseconds.
In any half bridge circuit (including the two halves of a full bridge circuit), when one of the power switching devices is turned on it applies a rapid change of voltage across the other device, which should already be turned off at this point if a short circuit across the power supply is to be avoided. It is common for this rapid change to be coupled through inter-electrode capacitances to the gate of the “off” device. If the signal coupled to the gate is large enough to turn the device on then both devices will be on at the same time and the resulting short circuit will cause a large amount of current to flow through the devices, in many cases resulting in the destruction of both devices. One of the most effective means to prevent this is to ensure that during the off period the gate of the switching device is held at a voltage sufficiently far away from the turn-on threshold of the device that the voltage coupled through the inter-electrode capacitances does not cause the gate voltage to cross the turn-on threshold. For MOSFETs and IGBTs this means that during the off period the gate should be held at a negative voltage with respect to the source or emitter electrode. The magnitude of the negative voltage applied to the gate depends on the expected size of the voltage change coupled through the inter-electrode capacitances.
Most inverters that are designed to draw their power from batteries use a nominal DC supply voltage between 12 and 48V. This DC supply voltage is classed as safe to touch by electrical safety organisations and requires minimal insulation of the supply conductors and terminals. Conversely, the voltages present in the high voltage side of the inverter is classed as hazardous and must be provided with a prescribed amount of insulation and isolation from any conductor that can be touched by a person in order to meet the relevant electrical safety regulations. In the field of electrical safety two circuits that have no conductive path between them are said to be galvanically isolated. Also, the distance through the air from a conductor in a hazardous voltage circuit to a conductor in a safe to touch circuit is known as a clearance distance and the distance between these two points measured along the surface of an insulator is known as a creepage distance.
It is common for the control circuit of the inverter to be powered from the DC supply voltage. The control circuit generates the switching signals required to operate the inverter and applies these signals to the gate drive circuits which generate the voltages and currents necessary to turn the power switching devices on and off. Because the control circuit is connected to the DC supply voltage it is necessary for the gate drive circuits that control the high voltage switching devices to provide isolation that meets the requirements of the electrical safety regulations. The regulations mandate such things as the minimum thickness of insulation and the minimum clearance and creepage distances. In addition, many of these regulations require that isolation be maintained even after one fault has developed in the equipment. For MOSFET or IGBT switches utilising gate drive circuits where the isolation is provided by a transformer the fault that is often considered is a short circuit from the drain or collector of the switching device to its gate. This has the potential to cause a large current to flow through the winding on the high voltage side of the gate transformer, potentially over-heating it and melting the insulation between the high voltage side and the control circuit side of the transformer. It is often desirable for the gate drive circuits that control the low voltage switching device to provide galvanic isolation but there is normally no need to comply with electrical safety regulations. The inverter depicted in FIG. 1 represents one possible design for a DC to AC inverter.
There are many other possible designs. One feature that they all have in common is the large number of power switching devices. This is especially true of inverters that use pulse width modulation to produce substantially pure sine wave outputs. In addition, each power switch in the low voltage section may be constructed from two or more individual switching devices connected in parallel and each power switch in the high voltage section may be constructed from two or more individual switching devices connected in series. This all adds up to a large number of switching devices and a similarly large number of gate drive circuits. Thus it is necessary for each gate drive circuit to be as simple as possible or else the cost of and the space required by all the gate drive circuits together would be unacceptable. Many known isolated gate drive circuits require a power supply for the circuitry on the switching device side of the circuit. Often this leads to a separate power supply being required for each power switching device. The power supplies for the high voltage devices must meet the same isolation standards as the gate drive circuits themselves. These factors often lead to a very complex design if gate drive circuits requiring separate power supplies are used.
U.S. Pat. Nos. 3,377541, 3,626,244, 3,760,285, 4,070,663, and 6,853,570 disclose circuits that use a resonating inductor to reduce the power required to repeatedly charge and discharge a capacitor. However, all these circuits require one or more power supplies to be connected to the circuit containing the capacitor and are therefore not suitable for producing a simple isolated gate drive circuit for a low standby power DC to AC inverter.
U.S. Pat. Nos. 4,443,719, 5,138,515, and 5,786,687 disclose circuits that use short pulses coupled through a transformer to turn a power switching device on and off. However, none of these circuits use resonance to reduce the power required to repeatedly charge and discharge the gate capacitance.
Only the circuit in U.S. Pat. No. 4,443,719 can hold a negative voltage on the gate during the off interval. However, this circuit uses the breakdown voltage of zener diodes to hold both the positive and negative voltages on the gate. As a result this circuit would require more power to drive the gate of a power switching device than most other circuits as the positive and negative pulses must have a magnitude that is large enough to break down the zener diodes and then charge or discharge the gate capacitance to the required voltage. For example, a typical high voltage MOSFET requires at least 10V on its gate to fully turn on. A circuit that charges the gate capacitance of one of these MOSFETs via a diode and discharges it via a zener diode would need to use a 10V zener diode in order to prevent the gate capacitance from discharging through the zener while the MOSFET is switched on. The drive circuit would then need to supply a positive 10V pulse to turn the MOSFET on and a negative 10V pulse to turn it of A circuit that used a 10V zener in one direction connected in series with a 5V zener in the other direction would be able to hold +10V on the gate when the MOSFET is on and −5V on the gate when it is off, but would need +15V pulses to turn the MOSFET on and −15V pulses to turn it off. Some low voltage MOSFETs can be fully turned on by a gate voltage of 5V. For these devices all the voltages given in the preceding example should be halved.
In addition, none of the circuits in U.S. Pat. Nos. 4,443,719, 5,138,515, or 5,786,687 have provision to prevent a short circuit in the power switching device from damaging the gate drive transformer. All these shortcomings prevent these circuits from being used to produce a simple isolated gate drive circuit for low standby power DC to AC inverters.